Group-based board moderation

Here is the complete group-based board moderation functionality.

Features include:
* Assign/manage moderator groups from the Manage Boards screen
* Moderator groups inherit any additional permissions from the Moderator group on boards they're assigned to moderate
* Moderator groups are listed in the moderators list on display and messageindex
* Moderator groups are listed in the "Boards" report in the admin center
* Users are listed as being able to moderate boards that groups they're in can moderate in the "Staff" report in the admin center
* Groups are shown as having any permissions inherited from "Moderator" in the "Board Permissions" report in the admin center
* Members in groups assigned to moderate a board are treated as if they were a locally-assigned moderator (including being shown as "Moderator" in posts in boards which they can moderate)
* "Track Permissions" page on user's profile shows any additional board-level permissions inherited from Moderator ("Given by: Moderator")